1. Reference to Related Provisional Application
This application claims the benefit of Provisional Application Ser. No. 60/023,069 filed Aug. 2, 1996.
2. Field of the Invention
This invention relates to a disconnectable vessel mooring system in which the vessel includes a structure for mounting a turret about which the vessel may weathervane when the turret is anchored to the sea floor by a spider buoy releasably connected to the turret. The spider buoy is buoyant and is anchored to the sea floor by a plurality of spaced mooring lines.
3. Description of the Prior Art
Reference is made to U.S. Pat. No. 5,306,186 dated Apr. 26, 1994, the entire disclosure of which is incorporated within this application as a written description for all purposes. As shown in the '86 patent, the spider buoy has a plurality of anchor legs comprising chains which are connected to the sea floor by anchors such as piles or drag embedment anchors. The anchor legs in the docked position of the buoy remain connected directly to the buoy. Anchoring forces are transmitted to the buoy. The spider buoy which is also referred to as a "mooring buoy" or "mooring element" is pulled tightly against the bottom of the turret below sea level by a mooring chain. The spider buoy is detachably latched to the turret. Thus, the turret is substantially fixed to the sea floor with the spider buoy docked against the turret. Any connections of the spider buoy to the turret, as disclosed in the '86 patent, must be made below the sea level.
Drilling and production operations are being performed at increased water depths and in even more harsh environments that include the potential passage of large icebergs. In instances where the water depth is great or the anchor legs are heavy to meet other design considerations, the design of the spider buoy and its connecting hardware becomes increasingly complex and expensive.
A spider buoy supports risers which extend to wells on the sea floor for the transport of hydrocarbon product to storage areas in the vessel. Connections of product lines on the turret are made to the risers in the buoy. Because the buoy is normally docked on the lower end of the turret, the buoy is below the level of sea water within the moon pool or vertical opening in the hull of the vessel which receives the turret. As a result, any coupling or connection of product lines from the turret to the risers of the buoy is performed beneath the sea level.
Mooring lines for the buoy are commonly formed of metal chains which may be of great weight, particularly at large water depths over thirty-five hundred (3500) feet, for example. Thus, a spider buoy when detached from a turret and supporting a plurality of heavy mooring lines or chains may submerge to a depth greater than desired. Otherwise the buoy must have great buoyancy to support the anchor legs. It is often uneconomical to provide a buoy of sufficient size and buoyancy to support the anchor legs or mooring lines when the buoy is detached from the turret and submerges to a suitable water depth for equilibrium.
A marker or spotter buoy floating on the surface of the sea water is usually connected to the submerged spider buoy to indicate the location of the submerged spider buoy. A small retrieval line extends from the marker buoy to the submerged spider buoy. A tanker or storage vessel locates the marker buoy and retrieves the spider buoy from its submerged position. The spider buoy is submerged at a depth low enough so that passing ships do not interfere. A suitable submerged depth is generally between thirty-five (35) and one hundred and fifty (150) meters. Thus, it is desirable to provide a disconnectable mooring system so that the spider buoy when disconnected from the turret is submerged to a desired depth range.